Singlet Ground State and Excitation Gap in the Double Spin Chain System KCuCl3

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Abstract
The magnetic properties of the S =1/2 double spin chain system KCuCl 3 have been investigated by susceptibility measurements using single crystals. It is found that the susceptibilities for three different external field directions exhibit broad maxima at around 30 K and decrease exponentially to zero with decreasing temperature. This result indicates that the ground state of KCuCl 3 is nonmagnetic and there is a finite gap in its excitation spectrum. This behavior is attributable to the quantum effect characteristic of the double spin chain. The structure of the exchange couplings in the double chain is discussed. Applying the recent spin ladder theory, we estimate the magnitude of the excitation gap to be Δ/ k B =35 K.